The experiment uses the high purity H4 electron beam at the CERN SPS (beam energy 100 GeV in 2017 and 150 GeV in 2018).
The backgrounds coming from the beam are further significantly suppressed by using the synchrotron radiation detector (SRD) to identify
electrons [1]}. This suppression factor for the hadron contamination of the beam is \(\sim 10^{-4}\) .
The most important subdetectors in this setup are the two electromagnetic (EM) calorimeters: the compact target-calorimeter WCAL assembled
from the tungsten and plastic scintillator plates with wavelength shifting fiber read-out and ECAL, a matrix of \(6\times 6\)  shashlik-type
lead - plastic scintillator sandwich modules [1]}. We also use a veto counter \(W_2\)  placed immediately after the WCAL and
a decay counter \(S_4\)  installed downstream the vacuum decay tube. Measuring the energy deposition in \(W_2\)  ensures that no charged particle exits
from the WCAL, while a signal compatible with two Minimum Ionizing Particle (MIP) in \(S_4\)  indicates that a decay to \(e^+e^-\)  happened in the decay volume.
The high efficiency thick (5cm) counter VETO and the hadron calorimeter HCAL are installed downstream the ECAL.
The HCAL consists of four modules, three of them are placed at the axis of the beam deflected by the MBPL magnets. They are
usually used as a veto against electroproduction of hadrons in the WCAL. The fourth module serves as a veto against upstream interactions of electrons
before reaching the target. Some most important distances of the setup are shown in Table REF . The distances in the invisible
mode configuration in 2016, 2017 were slightly different, this was taken into account in the exact signal yield calculation, which can be made only
using the detailed simulation.
<TABLE>